Lake Effect Snow
 
Lake-effect snowfall is an example of intense snowfall falling from convective clouds that form over a warm body of water. When a cold air mass passes over a much warmer body of water, it becomes unstable, with the warm moist air close to the water surface rising through the cold air mass above to produce convective snow clouds. These can form bands of snow showers that dump huge amounts of snow over over the nearby land, sometimes leading to severe localised disruption. 

The phenomenon derives its name from the large bands of snow showers that sometimes form over the Great Lakes of North America, although the same effect can be observed over any (relatively) warm body of water, such as the North Sea. The snowfall is most intense during the late autumn and early winter, when the temperature gradient between the water surface and the cold air above is at a maximum. Therefore, the instability and depth of the convective clouds is greatest and the snowfall is most intense. The large instability can allow cumulus congestus clouds to form, that are typically 1-4 km deep. 

 
Conceptual model of lake effect snow, illustrating the typical formation of convective clouds over a warm body of water, and the subsequent precipitation of heavy snow downwind.  
Credit: COMET/ MetEd. 

Formation of Lake Effect Snow

The temperature difference between the water and 850 hPa temperature must be greater than 13˚C for significant lake-effect snow to develop, and the presence of a cold polar/ Arctic air mass overflowing the lake or sea is essential. The warm water must lead to significant instability in the atmosphere, as well as leading to a modification and saturation of the air close to the water surface. 

  • The winds in the bottom 3 km of the atmosphere must be fairly unidirectional: with a directional shear of less than 30˚ required for snow banding to occur. 
  • Wind speeds need to be weak enough across the lake for moisture convergence to occur. Winds that are too strong will inhibit the amount of evaporative moisture available to produce heavy snow. The best combination is a deep layer of Arctic air moving between 10 and 40 mph. 
  • The fetch must be greater than 150 km (i.e. the distance that the wind blows across the water surface). More snow falls on areas in which cold air travels over longer stretches of water. 
  • After moving across the lake or sea surface, enhanced friction over the land surface slows down the modified air, creating a natural low-level convergence zone just inland from the prevailing flow, leading to enhanced ascent and precipitation.
  • This ascent can also be enhanced by elevated orography just inland from the prevailing flow. 
  • The atmosphere must be below freezing throughout it's entire depth (otherwise it would be lake effect rain)! 

Although the convective clouds will be below freezing, the air just above the warm lake will be above freezing, meaning that rain is possible above the water surface (or perhaps snow for particularly intense events). However, over the land downwind from the lakes, the entire atmospheric profile is likely to be below freezing.  
Lake effect snowbands on Lake Erie, taken from Buffalo, NY, during unprecidented snowfall that fell there on November 18th 2014. The cloudbase is at the surface. 
From: https://www.pinterest.com/annylouie/snowvember/ 
Atmospheric sounding of a lake effect snow event in Buffalo (at a similar time to the above photo). The steep lapse rates and high moisture content in the lowest 700 hPa promote the development of convective snow clouds. 
Credit: University of Wyoming.
Snow Banding

Lake effect snow tends to organize into bands across the lake surface. Single band development tends to occur when the atmosphere is absolutely unstable, so that there is a vigorous vertical transport of heat and moisture. The formation of cumulus congestus clouds releases latent heat, which warms the air greatly compared to the surrounding atmosphere. This causes the formation of a warm core shower structure, with a meso-low at the surface and a meso-high aloft - in a similar fashiion to a tropical cyclone. Horizontal thermal convergence into the low below the cloud base and divergence aloft leads to strong mesoscale ascent within the snowband. Indeed, the deeper the clouds, the stronger the meso-low and meso-high and the greater the ascent within the cloud band. Snow bands are generally 1 - 20 km wide and several hundered kilometers long, and can produce snowfall rates of up to 15 cm/hr.  

Multiple band development tends to occur when atmospheric instability is lower, with each band tending to be weaker that a single band. Therefore, they are more common when the fetch of the wind is much smaller - i.e. when the wind blows at right angles to the long axis of the Great Lakes. The temperature difference between each band and the environment is small, meaning that the mesoscale ascent within the bands is weak. 


A single lake effect snow band over Lake Michigan.
Credit: NASA/ MODIS.
Multiple lake effect snow bands forming on Lake Superior and Lake Michigan.
Credit: NASA/ MODIS.